The present invention relates to the problem of supplying accurately identical volumes of hydraulic fluid to two hydraulic actuators, or servo motors, so as to accurately synchronize their movement.
There are many situations where the movement of two elements or devices must be accurately synchronized and where hydraulic actuators are used to move the elements. In aircraft, for example, the thrust reversers which reverse the direction of thrust of the jet engines, for braking the craft on landing, are operated by hydraulic actuators. The movement of the actuators must be accurately synchronized so that the thrust reversers are moved in synchronism even though the loads on the actuators may differ. Various aircraft control surfaces, such as leading edge flaps, trailing edge flaps, and others which are hydraulically actuated must also be accurately synchronized. The problem is not limited to aircraft, of course, as there are many other types of equipment operated by hydraulic actuators in which the movement of two devices, such as machine elements, clamps, and other jointly movable devices, must be accurately synchronized.
Hydraulic systems of the type used for this purpose consist in general of a suitable source of high-pressure hydraulic fluid with valve means suitably arranged and controlled to supply the hydraulic fluid to two separate hydraulic actuators. In the types of equipment mentioned above, where it is necessary to synchronize the movements of the two actuators as closely as possible, synchronizing means must also be provided so that the devices to be moved by the actuators will move in synchronism with each other. The varying conditions of the actuators, such as different loads and dimensional differences, make it very difficult to achieve the desired accuracy which requires supplying exactly equal amounts of fluid to both actuators.
Various expedients have been proposed heretofore for synchronizing the movements of two hydraulic actuators. For example, it has been attempted to attain an accurate division of flow by causing the hydraulic fluid to flow through orifices. The actuators to be synchronized, however, often have different loads so that the back-pressure on the orifices is different and the flows differ correspondingly, frequently by more than 5%. Another method of synchronizing which has been used involves the provision of two auxiliary actuators each having equivalent volume to the actuators to be synchronized. The auxiliary actuators are mechanically tied together so that their simultaneous movement supplies equivalent volume to each of the actuators to be synchronized. The auxiliary actuators must, therefore, be of equal size to the actuators to be synchronized, so that this expedient requires as much additional space as the actuators themselves as well as the additional weight of the auxiliary actuators. Still another proposed method is to arrange the actuators to be synchronized in series in such a way that hydraulic fluid from one of the actuators drives a second actuator and they move together in synchronism. This arrangement, however, requires that the first actuator be twice the size of the second actuator, so as to have sufficient capacity for the hydraulic fluid, and the weight and size of the complete equipment are correspondingly increased. Both of these latter methods, therefore, are especially undesirable for aircraft use because of the increased space required and the severe weight penalty involved, neither of which are acceptable.